In many low- and middle-income countries, providers without formal training are an important source of antibiotics, but may provide these inappropriately, contributing to the rising burden of drug resistant infections. Informal providers (IPs) who practise allopathic medicine are part of India's pluralistic health system legacy. They outnumber formal providers but operate in a policy environment of unclear legitimacy, creating unique challenges for antibiotic stewardship. Using a systems approach we analysed the multiple intrinsic (provider specific) and extrinsic (community, health and regulatory system and pharmaceutical industry) drivers of antibiotic provision by IPs in rural West Bengal, to inform the design of community stewardship interventions. We surveyed 291 IPs in randomly selected village clusters in two contrasting districts and conducted in-depth interviews with 30 IPs and 17 key informants including pharmaceutical sales representatives, managers and wholesalers/retailers; medically qualified private and public doctors and health and regulatory officials. Eight focus group discussions were conducted with community members. We found a mosaic or bricolage of informal practices conducted by IPs, qualified doctors and industry stakeholders that sustained private enterprise and supplemented the weak public health sector. IPs' intrinsic drivers included misconceptions about the therapeutic necessity of antibiotics, and direct and indirect economic benefits, though antibiotics were not the most profitable category of drug sales. Private doctors were a key source of IPs' learning, often in exchange for referrals. IPs constituted a substantial market for local and global pharmaceutical companies that adopted aggressive business strategies to exploit less-saturated rural markets. Paradoxically, the top-down nature of regulations produced a regulatory impasse wherein regulators were reluctant to enforce heavy sanctions for illegal sales, fearing an adverse impact on rural healthcare, but could not implement enabling strategies to improve antibiotic provision due to legal barriers. We discuss the implications for a multi-stakeholder antibiotic stewardship strategy in this setting.